Diesel exhaust system including NOx-trap

ABSTRACT

An exhaust gas treatment system ( 8 ) for a diesel engine ( 10 ) having a NOx absorber ( 28 ) charged with solid absorbent for absorbing NOx from a diesel exhaust gas and means ( 18 ) for introducing intermittently into the exhaust system CO upstream of said absorber ( 28 ) whereby the solid absorbent is regenerable by contact with the CO-enriched gas. A process for treating NOx in a diesel exhaust gas comprises absorbing the NOx in a solid absorbent and regenerating the absorbent by contacting it with CO-enriched gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase application of PCTInternational Application No. PCT/GB01/02430.

FIELD OF INVENTION

The present invention relates to an exhaust gas treatment system for adiesel engine, and in particular to an exhaust system including aNOx-trap. Furthermore, the invention concerns a process forregenerating, i.e. removing absorbed NOx from, a NOx-trap in a dieselexhaust system.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND/OR DEVELOPMENT

Not applicable.

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON COMPACT DISC (See 37 C.F.R. §1.52(e)(5))

Not applicable.

BACKGROUND OF THE INVENTION

Engine-out emission of carbon monoxide (CO), hydrocarbons (HC) andnitrogen oxides (NOx) depends on the air-to-fuel ratio (A/F), defined byequation (1):A/F=mass of air consumed by the engine/mass of fuel consumed by theengine  (1).The A/F where there is just enough air to complete the combustion of allhydrocarbons in the fuel is known as stoichiometry, and is at 14.7 ingasoline engines. If the A/F is below this value, then the engineoperates under excess fuel conditions, leading to incomplete fuelcombustion. The exhaust gas will then contain more reducing reactants(CO, HC) than oxidising reactants (O₂, NOx), and is called rich. If theA/F exceeds 14.7, then the engine operates under excess air conditions,giving rise to an exhaust gas that contains more oxidising reactantsthan reducing reactants, and the exhaust gas is called lean.

A common way of classifying the engine-out exhaust gas composition isthe lambda (λ) value, defined by equation (2):λ=actual engine A/F/stoichiometric engine A/F  (2)From equation (2) it will be seen that when the exhaust gas compositionis lean, λ≧1, and when the exhaust gas composition is rich, 1≧λ.

In order to control NOx in exhaust gases from lean-burn gasolineengines, there has been devised a NOx absorber/catalyst which storesNOx, e.g. as nitrate, when an engine is running lean. In astoichiometric or rich environment, the nitrate is understood to bethermodynamically unstable, and the stored NOx is released and isreduced by the reducing species present in the exhaust gas. This NOxabsorber/catalyst is commonly called a NOx-trap. By periodicallycontrolling the engine to run stoichiometrically or rich, stored NOx isreduced and the NOx-trap is regenerated.

A typical NOx-trap formulation includes a catalytic oxidation component,such as platinum, a NOx-storage component, such as barium, and areduction catalyst e.g. rhodium. One mechanism commonly given forNOx-storage during lean engine operation for this formulation is: (i)NO+½O₂→NO₂; and (ii) BaO+2NO₂+½O₂→Ba(NO₃)₂. In the first step, thenitric oxide reacts with oxygen on active oxidation sites on theplatinum to form NO₂ by the storage material in the form of an inorganicnitrate.

When the engine runs under rich conditions or at elevated temperatures,the nitrate species become thermodynamically unstable and decompose,producing NO or NO₂ according to equation (iii) below. Under richconditions, these nitrogen oxides are subsequently reduced by carbonmonoxide, hydrogen and hydrocarbons to N₂, which can take place over thereduction catalyst. (iii) Ba(NO₃)₂→BaO+2NO+{fraction (3/2)}O₂ orBa(NO₃)₂→BaO+2NO₂+½O₂; and (iv) NO+CO→½N₂+CO₂ (and other reactions). Inthe reactions of (i)-(iv) above the reactive barium species is given asthe oxide. However, it is understood that in the presence of air most ofthe barium is in the form of the carbonate or possibly the hydroxide.The above reaction schemes can be adapted accordingly for species ofbarium other than the oxide.

Using sophisticated engine management techniques and known fuelinjection components such as common rail, it is now becoming possible toadopt NOx-trap technology into the exhaust treatment systems for dieselengines. See, for example, EP-A-0758713 described below.

DESCRIPTION OF THE RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37C.F.R. §§ 1.97 AND 1.99.

EP-A-0341832 (see also U.S. Pat. No. 4,902,487) describes a process forremoving soot from diesel exhaust gas containing NOx by passing the gasunfiltered over an oxidation catalyst to convert NO to NO₂, collectingthe soot on a filter and using the NO₂-enriched gas to combust thecollected soot, the amount of NO converted to NO₂ being sufficient toenable the combustion to proceed at a temperature less than 400° C.

The process described in EP-A-0758713 adopts the process disclosed inEP-A-0341832 and further includes the step of removing NOx from thecombustion outlet gas by means of a solid absorbent and regenerating theabsorbent by intermittent contacting it with rich exhaust gascomposition.

It has been proposed to remove NOx from diesel exhaust gas by reactingit catalytically with injected ammonia. This process is generally calledselective catalytic reduction (SCR) using ammonia. See, for example,WO-A-99/39809. Ammonia SCR does not necessarily require the exhaust gascomposition to be made rich or equivalent, but it does require theaddition of a reductant to the exhaust gas.

BRIEF SUMMARY OF THE INVENTION

We have now found that CO is effective to regenerate a NOx absorbent ina diesel exhaust system. More particularly, we have found that theconditions for CO-promoted regeneration are approximately the same as inammonia SCR and a net-lean diesel exhaust gas composition is preferred.

According to one aspect, the invention provides a process for treatingNOx in a diesel exhaust gas comprising absorbing the NOx in a solidabsorbent and regenerating the absorbent by contacting it withCO-enriched gas.

According to a preferred embodiment, the CO-enriched gas is produced byintermittently increasing the CO content of the exhaust gas, e.g. byinjection into a conduit carrying it.

The gas contacting the absorber typically contains 1-20% v/v of CO. Thisusually suffices to provide a 20-100-fold excess over the total numberof oxygen atoms present in the NOx leaving the absorbent duringregeneration. Suitably the lambda value of the CO-enriched gas (alsoreferred to herein as the “redox composition” of the gas) is in therange 0.7 to 1.5 lambda, especially 1.0 to 1.2 lambda. It preferablycontains at least enough, especially 1.5 to 3 times, the concentrationof O₂ to oxidise all the CO and other combustibles present. Intermittentincrease of CO content need not attain net-rich gas composition.

According to a further aspect, the invention provides an exhaust gastreatment system for a diesel engine comprising a NOx absorber chargedwith solid absorbent for absorbing NOx from a diesel exhaust gas andmeans for introducing CO intermittently into

the exhaust system upstream of the absorber whereby the solid absorbentis regenerable by contact with the CO-enriched gas.

According to a preferred embodiment, the exhaust system furthercomprises, upstream of the NOx absorber, a catalyst effective to promoteoxidation of at least NO to NO₂ and a filter effective to collect dieselsoot from the exhaust gas and hold it for combustion reaction with theNO₂ in the gas. In this embodiment, the present invention complementsour Continuously Regenerating Trap (CRT™) technology which is describedin our EP-A-0341832.

Preferably, the NOx absorbent comprises one or more of: (a) compounds ofalkali metals, alkaline earth metals, rare earth metals and transitionmetals capable of forming nitrates and/or nitrites of adequate stabilityin absorbing conditions and of evolving nitrogen oxides and/or nitrogenin regenerating conditions; or (b) adsorptive materials such aszeolites, carbons and high surface-area oxides, or mixtures of any twoor more thereof.

Such a system preferably comprises a catalysed absorbent. By ‘catalysed’is meant that the absorbent is intimately associated with catalyticmaterial effective for the reaction of CO with NOx. Such material may befor example co-precipitated or co-impregnated or co-deposited with NOxabsorbent or present as one or more sandwiched layers or serial zones oras fine (e.g. 10-500 microns) particles on or in a layer of absorbent oramong particles of absorbent. Whether catalysed or not, the absorbentmay be provided in one unit or a succession of separate units. It istypically on a honeycomb substrate, such as a single honeycomb ormultiple honeycombs.

Compounds (a) may be present (before NOx absorption) as compositeoxides, e.g. of alkaline earth metal and copper such as Ba—Cu—O orMnO₂—BaCuO₂, possibly with added Ce oxide, or Y—Ba—Cu—O and Y—Sr—Co—O.(For simplicity the oxides are referred to, but in situ hydroxides,carbonates and nitrates are present, depending on the temperature andgas composition). Whichever compounds are used, there may be presentalso one or more catalytic agents, such as precious metals, especiallyPGMs, effective to promote redox reactions between nitrogen oxides andCO.

The oxidation catalyst or the catalyst associated with the absorbent orfollowing it can be any that is active and stable. Typically thesecatalysts comprise one or more PGMs, especially Pt, Rh, Pd andcombinations thereof, on a high-surface area washcoat on a honeycombstructure. Detailed catalyst formulation is chosen according to whichduty in the system the catalyst is to carry out. Suitable catalysts havebeen described in the prior art and are available to skilled persons.

The catalysts and absorbent are suitably carried on a ceramic or metalhoneycomb substrate, the ceramic comprising one or more of alumina,silica, titania, cordierite, ceria, zirconia, silicon carbide or other,generally oxidic, material. The honeycomb carries a washcoat and, in oneor more layers thereon, the active catalytic and/or absorptive material.The honeycomb has typically at least 50 cells per square inch (cpsi),possibly more, e.g. up to 1000 cpsi, or up to 1200 cpsi if composedstructurally of metal. Generally the range 100-900 cpsi is preferred forthe catalysts and absorbent.

Desirably, the filter is capable of trapping the soot without causingexcessive backpressure in the system and engine upstream. In general,ceramic, sintered metal or woven or non-woven wire filters are usable,and wall-flow honeycomb structures may be particularly suitable. Thestructural material of the filter is preferably porous ceramic oxide,silicon carbide or sintered metal. A coating such as alumina and/or acatalyst such as La/Cs/V₂O₅ may be present. The soot is generally carbonand/or heavy hydrocarbons, and is converted to carbon oxides and H₂O.Certain embodiments of this principle are in commercial use in JohnsonMatthey's Continuously Regenerating Trap (CRT™) technology, and aredescribed in above-mentioned EP-A-0341832 (see also U.S. Pat. No.4,902,487), the teaching of which is incorporated herein by reference.

According to a preferred embodiment, the system may further comprise,downstream of the absorber, a catalyst system effective to promotereactions of HC and CO with O₂ to H₂O and CO₂ and preferably with NOx toN₂.

Advantageously, the system may further comprise sensors, indicators,computers and actuators, effective to maintain operation within desiredconditions. Preferably a means for controlling CO enrichment of theexhaust gas includes a computer which can be part of the enginemanagement unit if desired. Control of the system can be regulated withopen or closed feedback using information gathered from the sensors,indicators, etc. as explained below.

For regeneration of the NOx absorber the CO may be fed in as such,subject to effective precautions against leakage, or as one or morecompounds decomposable to CO in the conditions of the system, forexample formic acid. Compounds introducing reductant, for example formicesters such as methyl formate, may be used. If reductant such as dieselfuel or HC derived therefrom is present, its concentration by carbonatoms is less than the CO, especially less than 10% of the CO. The CO isintroduced preferably by engine inlet adjustment.

Preferably the means for controlling the regeneration of the absorberperforms one or more of the following illustrative techniques:

-   (a) injection responsive to ultimate detection of NOx leakage from    or “slip” past the NOx absorber;-   (b) injection responsive to prediction based on input of data on    deliberate or load responsive engine management variation; and-   (c) allowance for gas composition variations, for example non-steady    conditions such as incomplete warm-up or weather. Usually the    regeneration phase can be a small fraction of engine running time,    e.g. 0.1% to 5%, depending of course on operating conditions.

The required CO-rich gas is obtained, for example, by pilot injectiontechnique.

The control means may include sensors for at least one of: fuelcomposition; air/fuel ratio; exhaust gas composition (includingtail-pipe NO₂) and temperature at one or more points along the exhaustsystem; and pressure drop, especially over the filter. It may includealso indicator means informing the engine operator, computer meanseffective to evaluate the data from the sensor(s), and control linkageseffective to adjust the engine to desired operating conditions takingaccount of e.g. start-up, varying load and chance fluctuations.

In addition, the system may include routine expedients, for exampleexhaust gas recirculation (E.G.R); and means such as cooling, orelectric heating, to adjust the temperature of the gas to a levelpreferred for nearer optimum operation of downstream components.

According to a further aspect, there is provided a diesel engine havingan exhaust system according to the invention. The engine is preferablyof the direct injection common-rail type, especially using injectionpressures in the range 1000-2000 bar, and advantageously isturbo-charged.

The engine may be the motive power for a vehicle, or may be a stationarypower source or auxiliary power source. It may be for a ‘heavy duty’vehicle, e.g. at least 3500 Kg (as defined by the relevant European, USFederal or Californian legislation), or a ‘light duty’ vehicle,including in particular a passenger car or light van and likely to beoperated according to the ‘urban cycle’.

Desirably the engine is fuelled with low-sulphur fuel, i.e. having lessthan 50 ppm, especially less than 10 ppm, by weight as elemental S. Foroperation with higher sulphur fuels, a SOx absorbent may be used.

Most preferably, the engine according to the invention is operated incompliance with the European IV standard.

The system may be structured within a single housing (“can”), or inseparated housings according to engine design and under-floor or otherspace considerations. Thus for example for V-engine configurations, someor all of the elements of the system may be disposed in parallel.

In order that the invention may be more fully understood, reference ismade, by way of illustration only, to the Example below and theaccompanying drawing, which shows schematically a diesel engine equippedwith a preferred embodiment of an exhaust system according to theinvention. In the drawing, full lines represent flow of gas and dottedlines represent flow of information or control power.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic view of a diesel engine and an exhaust systemaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an exhaust system 8 for a diesel engine 10, whichengine having a common rail fuel feed 12 including valves 14 and highpressure pump 16 supplying diesel fuel, for example of under 50 ppmsulphur content. Exhaust system 8 comprises fuel feed 12 under thecontrol of computer 18 which computer is responsive inter alia to gascomposition at the outlet of catalyst 30 (described below) and topressure-drop across filter 26 (also described below), and is programmedto actuate valves 14 to inject fuel at the normal inlet stroke, and alsoto vary inlet conditions to produce CO-enriched exhaust intermittently.Engine exhaust gas passes via pipe 20 to can 22, at the inlet end ofwhich is catalyst 24, a low temperature light-off oxidation catalystsupported on a 400 cells/in² ceramic honeycomb. Catalyst 24 is designedto convert at least 70% of the NO in the normal gas to NO₂.

The gas leaving catalyst 24 passes into soot filter 26, which is of theceramic wall flow type, and collects soot particles over 50 nm. The NO₂and surplus oxygen in the gas oxidise the soot at temperatures around250° C. with no tendency to blocking.

Gas leaving filter 26 then enters NOx absorber 28 which includes alsoPGM catalytic material. During normal lean operation of the engine andwithout CO enrichment by 18, absorber 28 absorbs NOx from the exhaustgas while it has capacity so to do. When, however, gas enriched with COreaches it, the NOx is released and converted at least partly to N₂, forexample by action of a reducing catalyst, such as rhodium. The gas, nowstill containing CO, O₂ and possibly some NOx, passes into catalyst 30,where these reactants are brought substantially to chemical equilibriumcomprising less environmentally harmful gases.

The process and system of the invention is expected to be capable ofmeeting European Stage IV emission legislation.

EXAMPLE

A NOx absorber comprising a 400 cpsi monolith having wall thickness of{fraction (6/1000)} of an inch, measuring 1 inch in length by 3 inchesin diameter and carrying a coating containing 62.8% w/w alumina, 23.8%w/w ceria-zirconia mixed oxide, 9.9% w/w magnesia, 1.7% w/w platinum,1.67% w/w palladium, 0.167% w/w rhodium, was subjected for 520 secondsat 200° C. to a synthetic gas stream to imitate the exhaust of a dieselengine, but containing NOx at 500 ppm. It was then fully regenerated byswitching the gas feed to net-rich for 180 seconds [‘rich ramp’] andoperated in this cycle at the same temperature and flow rate: 5 secondsregeneration phase in CO-enriched gas; 60 seconds absorption phase inuntreated lean gas.

The gas compositions at the absorber inlet and the NOx concentrations atrelevant points are shown in Tables 1 and 2 respectively.

TABLE 1 Rich ramp lean storage lean regeneration Lambda 0.96 1.39 1.20H₂O 10 10 10 CO₂ 10 10 10 O₂ 0.5 6.2 6.2 CO 2.0 0.04 5 H₂ 0.67 0.01 0.01C₃H₆ 100 ppm 100 ppm 100 ppm NOx 500 ppm 500 ppm 500 ppm

TABLE 2 NOx ppm at absorber outlet Rich-regenerated: start 1 end 300Cycle 1: start 150 end 320 Cycle 2: start 100 end 360 Cycle 3: start 100end 390

It is evident that CO is capable of substantial regeneration of theabsorber, and has the potential, after optimisation, for comparabilitywith rich regeneration.

1. A process for treating NOx in a diesel exhaust gas comprising CO, NOx, soot, HC and O₂, which process comprising catalysing oxidation of NO to NO₂, collecting the soot on a filter, combusting the soot by reaction with the NO₂, absorbing NOx present in the gas in a solid regenerable absorbent and intermittently regenerating the absorbent by increasing the CO content of the exhaust gas and contacting said absorbent therewith, wherein the redox composition of the CO-enriched exhaust gas is in the range of 1.0 to 1.2.
 2. A process according to claim 1, wherein the CO content of the exhaust gas is increased by engine inlet adjustment.
 3. A process according to claim 1, wherein the CO content of the exhaust gas is increased by introducing CO per se or a CO precursor compound into the exhaust gas.
 4. A process according to claim 1, wherein the CO content of the regenerating gas is in excess (by carbon atoms) over all other carbon-containing reductants present.
 5. A process according to claim 1, wherein the CO-enriched gas contains CO in a 20-100 fold excess over the total number of oxygen atoms present in the NOx leaving the absorbent during regeneration.
 6. A process according to claim 1, wherein the CO-enriched gas contains at least enough O₂ to oxidise all the CO and other combustibles present.
 7. A process according to claim 1, wherein the gas is the product of combustion of a fuel containing less than 50 ppm w/w of sulphur.
 8. A process according to claim 1, wherein the CO content of the exhaust gas is increased by introducing a CO precursor compound into the exhaust gas, wherein the CO precursor compound is formic acid or esters of formic acid.
 9. A process according to claim 8, wherein the CO precursor compound is methyl formate.
 10. A diesel engine comprising an exhaust gas treatment system comprising a catalyst effective to promote oxidation of at least NO to NO₂; a filter disposed downstream of the oxidation catalyst effective to collect diesel soot from the exhaust gas and hold it for combustion of the soot, wherein the combustion of the soot is by reaction with the NO₂ in the gas; a NOx absorber charged with solid absorbent for absorbing NOx; and means for introducing CO intermittently into the exhaust system upstream of the NOx absorber to produce a CO-enriched gas of redox composition of from 1.0 to 1.2 lambda, wherein the solid absorbent is regenerable by contact with the CO-enriched gas.
 11. An exhaust system according to claim 10, wherein the CO introducing means comprises means for adjusting the engine inlet.
 12. An exhaust system according to claim 10, wherein the CO introducing means comprises means for introducing CO per se or a CO precursor compound into the exhaust system.
 13. A diesel engine according to claim 10, wherein the solid absorbent comprises at least one of: (a) an alkali metal compound, an alkaline earth metal compound, a rare earth metal compound and a transition metal compound adapted for forming a nitrate, adapted for forming a nitrite or adapted for forming both a nitrate and a nitrite, which nitrate or nitrite having adequate stability in absorbing conditions and being adapted for evolving nitrogen oxides, nitrogen or nitrogen oxides and nitrogen in regenerating conditions; (b) an adsorptive material; and (c) mixtures of (a) and (b).
 14. A diesel engine according to claim 10, wherein the NOx absorber is catalysed.
 15. A diesel engine according to claim 10, further comprising sensors, indicators, computers and actuators, effective to maintain operation within desired conditions.
 16. A diesel engine according to claim 10, which is of the direct injection common-rail type.
 17. A diesel engine according to claim 10, further comprising, downstream of the NOx absorber, a catalyst system effective to promote reactions of HC and CO with O₂ to H₂O and CO₂.
 18. A diesel engine according to claim 17, wherein the catalyst system is further effective to promote reaction of HC and CO with NOx to N₂.
 19. A diesel engine according to claim 10, operated in compliance with the European IV standard.
 20. A turbo-charged diesel engine according to claim
 19. 